The present invention relates generally to image compression and more specifically to a technique for differentially compressing portions of the image.
Emerging applications, such as full color (24-bit) desktop publishing, photovideotex, phototransmission, supported by 24-bit color scanners, monitors, printers, and cameras, need data reduction and standards to reach acceptable price performance levels.
One initial focus in the early 1980's, was on the use of photographic images within videotex systems. It was expected that such systems would eventually employ ISDN (64 Kbit/sec) lines for transmission and monitors as softcopy displays. The initial algorithm requirements and the evaluation procedures reflect the early focus. 720 by 576 pixels color images (CCIR 601 format) were selected as test material. Compression goals included good image quality around 1 bits/pixel and "progressive build-up", allowing an early recognition of the image at a lower quality. The image quality evaluation was tied to relatively inexpensive monitors.
In 1986 the Joint Photographic Expert Group (JPEG) was formed as a joint committee of ISO and the CCITT to develop and propose an efficient image compression standard. Soon more target applications were identified, broadening the scope of the standard. It needed to support a wide variety of pixel resolutions, color spaces, and transmission bandwidths. In addition the efficiency of implementation in both software and hardware became an important additional requirement.
Given these requirements, in a competition between twelve algorithms covering a wide spectrum of algorithms, three finalists were selected in June, 1987: block truncation approach, an interpolative spatial domain approach and a transform based method. From these three finalists, the Adaptive Discrete Cosine Transform (ADCT) was unanimously selected in early 1988 as having produced the best picture quality. Since then, a cooperative effort to refine, test and document the DCT-based method has been in progress.
This effort resulted in development of a three part structure, including the baseline system, the extended system, and independent lossless coding function. Technical agreement was reached in the October, 1989 Tokyo meeting, a draft specification was made publicly available in January, 1990, and an ISO Draft Proposed Standard (DPS) is expected for 1991.
The selected baseline algorithm is a lossy technique based on the discrete cosine transform (DCT), a uniform quantizer, and entropy encoding. The transform removes the data redundancy by concentrating most of the information in the first few transform coefficients. The quantizer controls the loss of information and the picture quality. The entropy encoding reduces the entropy of the signal. The JPEG proposed standard includes a baseline system, an extended system, and a separate lossless function. The baseline system represents the default communication mode, and each standard decoder is required to interpret data coded with the baseline system. The extended system provides additional features such as progressive build-up and arithmetic coding. These features can be used when implemented by both the encoder and the decoder.